1. Field of the Invention
This invention relates to compositions obtained by cooking a cationic or amphoteric starch and a cationic, anionic, nonionic, or amphoteric polyacrylamide. The resulting modified starch compositions have general utility as a clarifying aid for removing solids from an aqueous dispersion, and have special utility as a retention aid in the manufacture of paper.
2. Background of the Prior Art
Paper production involves the formation and dewatering of a web primarily composed of cellulose fibers and inorganic filler. The web is formed by spreading an aqueous suspension containing the cellulose fibers and inorganic filler over a wire or net, and then removing water to form a fiber web or sheet. The aqueous suspension is referred to as "paper furnish" in the trade, and the removed water is referred to as "white water."
The industry long has sought ways to reduce the percentage of small cellulose fibers and filler particles that are removed with the white water as the paper web is formed. Not only does this represent a loss of material, but it also contributes to a build-up of material in the white water known as "anionic trash" that impairs efficient operation of the equipment. Thus, improved retention of the smaller particles not only makes water removal easier, but also improve yield and productivity of the paper-making process.
Many additives have been suggested in the prior art for improving fines retention and wet-end drainage. Cationic starches frequently are used for this purpose, particularly the relatively expensive cationic potato and waxy maize starches. Less expensive cationic corn starch has been used, but it generally does not provide adequate fines retention and wet-end drainage.
Many suggestions are made in the prior art concerning improvements to the efficiency of cationic starch for this purpose. PCT application WO 91/07543 published May 30, 1991, for example, proposes that a cationic starch, a cationic polyacrylamide, and a polymeric silicic acid may be added to the cellulose suspension to improve fines retention and dewatering. The addition of large quantities of polyacrylamide, however, not only adds substantial costs to the paper-making process, but also may overflocculate the paper furnish, resulting in poor paper formation.
Thus, there is an ongoing need for additives to improve fines retention and wet-end drainage in the manufacture of paper.